This is a very similar situation to me. After I split up with my girlfriend she admitted that whilst we were going out she was giving her ex-boyfriend blow jobs(!)
Now we are split up I still see her and she still sees her ex - but I seriously suspect she's now shagging him again. Whilst with me, she's changed to a completely platonic relationship. It's like the thought of doing anything sexual never crosses her mind. I should say we used to have a normal(?) sex life and if anything she was submissive rather than dominant.
I know, as with 'cover_my_wife' this is probably just cheating rather than cuckolding, but the thought of her actively blowing her ex-boyfriend when she knows she's going out with me is pretty much all I ever wank about now.
The biggest turn on is that when ever I get into a serious talk with her about getting back together, she references her cheating blow jobs as to why *I* would never want to. It always gets me rock hard to think that she might be leaving the door open for a cuckold-type relationship.
